The length of an object is usually measured by placing a ruler directly beside it and determining how many multiples of the ruler length fill out the length of the object. If the object is at a distance from the observer, neither this distance nor the object's length can be measured with a ruler in this direct way.
In general, elaborate measuring devices and/or procedures are necessary in the case just mentioned, where for example a light beam or laser beam is reflected from an object at a large distance from the source of the beam. The distance is then computed from the measured travel time of the beam and the speed of light.
It is important for the driver of a car to determine his distance from a moving vehicle in front of him, so that he can maintain an adequate safety distance that depends on the speed of his car. The safety distance is the distance needed by the driver for his braking response and actual brake application to prevent a collision with the vehicle in front of him should its driver make a sudden stop.
Since the distance between cars travelling one behind the other is relatively short, it is not in general possible to determine this distance by reflecting electromagnetic waves from the car in front.
The usual method in the past of determining the necessary safety distance has been the familiar "rule of thumb" as follows. The driver reads off the speedometer his current speed in kilometers per hour (km/h) and divides this number by two. The result, regarded as being in meters, is the minimum safety distance that must be maintained. The driver may in some situations be able to determine his distance from the car ahead by counting the number of lane divider lines (each one being 9 meters long on the freeway, or 6 meters off the freeway) or the number of guideposts (50 meters apart on straightaways) between him and the other car.
There is another method of estimating the safety distance. At the instant when the car in front is even with some prominent reference point (such as a bridge, tree or traffic sign), the driver begins counting to himself "one thousand, two thousand, . . . ". When the driver's car pulls even with the same reference point, the number of thousands that he has counted by then is the number of seconds that separate him from the car in front. From this elapsed time and his speed, the driver can calculate his distance from the car ahead.
All of these methods for determining distance between cars are clearly a burden on the driver, since continual glancing over at guideposts or counting lane divider lines or elapsed seconds, followed by mental calculations, may severely distract the driver's attention from traffic.